The present invention relates to a compressor capacity control method and device for controlling the capacity of a compressor by changing-over an unloading state to a loading state and vice versa.
In a conventional compressor-operation method, a capacity control method for controlling the flow rate of the gas (air) discharged from the compressor is performed so as to appropriately maintain the pressure of the gas on the loading side of the compressor. FIG. 7 explains an example cyclegraph as to a conventional capacity control method for a compressor. In general, during a loading state of a compressor, the flow rate of the air discharged from the compressor is greater than the air consumption on the loading side; accordingly, the pressure of the discharged air gradually increases during the loading state; and, in the conventional control method, the pressure of the discharged air is detected so that the pressure of the air in a piping on the loading side is maintained in a predetermined pressure range between an upper limit pressure and a lower limit pressure, by changing-over over the unloading state of the compressor to the loading state of the compressor and vice versa, in the pressure range, in response to the discharged air pressure.
A capacity control method as described above has been proposed; whereby, a loading/unloading state in a previous cycle (regarding load changeover repetition or cyclic pressure repetition) is observed so that the pressure to be controlled in a following cycle is computed, and the pressure control is performed; in the pressure control, the pressure level is moderately restrained within a range where the life time of the compressor is not deteriorated and an energy saving operation is realized.
For instance, JP2684715 discloses a method whereby a pressure sensor detects the change of the pressure on the air consuming side (the loading side); the loading/unloading repetition cycle time from an unloading state to the next unloading state via a full-loading state is grasped based on the detected change of the pressure; in a case where the grasped loading/unloading repetition cycle time is longer than a predetermined time span, an unload operation start-pressure (at which the compressor operation is changed-over toward an unloading operation) is reduced by an appropriate decrement within a predetermined allowable- and controllable-range; on the other hand, in a case where the grasped loading/unloading repetition cycle time is shorter than a predetermined time span, the unload operation start-pressure is increased by an appropriate increment within the predetermined allowable- and controllable-range; thus, the unload operation start-pressure is set closer to a pressure level of a full load operation; and, the stable pressure transition as a result saves energy (power) consumption.
Further, JP3125794 discloses a capacity control method whereby an on-off control valve is provided at the air suction side of the compressor; the on-off control valve changes-over a full-load operation to an unload operation (i.e. no load operation) and vice versa by use of the on-off action of the on-off valve, so that the air pressure on the discharged side of the compressor is kept within a range between an upper limit set-pressure Pmax (cf. FIG. 7) and a lower limit set-pressure Pmin (cf. FIG. 7); wherein, the upper limit set-pressure Pmax and the lower limit set-pressure Pmin are amended so that an on-off change-over period Δt regarding the on-off valve is not shorter than a minimum period Δtmin; thus, the on-off change-over period in which the full-load operation is changed into no load operation or vice versa can be equal to or longer than a prescribed period. Accordingly, frequent change-over between the load modes can be evaded even where the load burdened upon the compressor stays at a high load level.